This invention relates generally to methods for testing two-phase compressible compositions having gas under pressure. Examples of these compositions are foamed cementing compositions used in the oil and gas industry. This invention also relates to systems and devices which can be used in such methods.
A growing trend in the oil and gas industry is the use of foamed cementing compositions, which are made by foaming a Portland-based cementing composition with a gas (preferably an inert gas), a foamer and a foam stabilizing agent. Such compositions have mechanical property advantages and operability advantages over conventional, non-foamed Portland-based cementing compositions. For example, conventional compositions undergo shear failure whereas foamed cementing compositions undergo plastic failure under stress/stress cycle loading. Susceptibility to plastic failure as opposed to shear failure is preferred because of the prolonged well life expected or provided as a result of increased resiliency/stress resistance. Shear failure of an annular cement sheath generally results in loss of zonal isolation, annular pressure build-up, collapsed casing, etc.
With all types of cementing compositions, the ability to test them is desired. For example, a conventional composition can be tested in any of various types of equipment. Two particular test units are the MACS Analyzer and the triaxial load cell, both from Halliburton Energy Services. If both of these devices are used to test a conventional cementing composition, a sample of the composition is put into or made in a pressurizable chamber of the MACS Analyzer and tested there for properties such as compressibility, thickening time, and static gel strength. A portion of this sample or another sample of the composition can be allowed to harden into a core and separately put in the triaxial load cell for testing of mechanical properties such as Young's modulus and Poisson's ratio. It would be desirable to perform the same tests on foamed cementing compositions so that the same test data can be developed for this class of material. These tests can be performed as described above; however, there has been a shortcoming which can adversely affect the significance of such testing as applied to foamed cementing compositions, or other two-phase compressible compositions having entrained gas under pressure.
The entrained gas contained in the pore spaces within the matrix of the composition is allowed to escape to the atmosphere as a function of the specimen's permeability if it is not kept under suitable pressure. The shortcoming of prior test methods and systems is that they have not enabled a specimen to be made under pressure, such as in the MACS Analyzer, transferred under pressure to another test device, such as the triaxial load cell, and then tested under pressure whereby the two-phase integrity of the specimen is maintained throughout the entire process. Because the actual composition used in an oil or gas well will remain under pressure in the wellbore, such prior techniques which allow for the gaseous phase to escape before testing may not produce meaningful test data relative to the actual composition that will exist in the well. Accordingly, there is the need for a method and system for testing such a composition while maintaining the integrity of the test specimen.